Why free rotation about a `pi`-bond is not possible ?

Explanation

### Step-by-Step Solution:

1. **Understanding Bonds**:
- In chemistry, bonds between atoms can be classified into two main types: sigma (σ) bonds and pi (π) bonds.

2. **Formation of Sigma Bonds**:
- Sigma bonds are formed by the head-on or end-to-end overlap of atomic orbitals. This can involve s-orbitals, p-orbitals, or hybrid orbitals.
- The overlapping occurs in such a way that the electron density is concentrated along the axis connecting the two nuclei.
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